Historically, lead projectiles have been utilized with firearms. Factors that contributed to this choice include lead's relatively high density (11.3 g/cc), workability, and inexpensiveness. However, under certain conditions, environmental and/or wildlife regulations may preclude the use of lead as a projectile due to the toxicity thereof. For example, an animal might ingest the lead projectile, an animal that has been shot with a lead projectile might be consumed by another animal, and/or the lead might act as an environmental contaminant. In addition, lead fumes and dust have been shown to cause health problems in people who routinely practice target shooting at indoor ranges. For example, these health problems may be experienced by law enforcement personnel who are required to maintain continuous proficiency in marksmanship.
When considering alternative materials to be utilized, consideration should be made about the frangibility and ductility of the resulting projectiles. For example, for firing ranges, and especially indoor ranges, a frangible projectile may be desired so that shooters and observers are not subjected to ricocheting or rebounding bullet fragments created when bullets strike hard targets, such as steel plates, at relatively close range (e.g., 25 yards, about 23 m). In such applications, it may be desirable to utilize projectiles that are highly frangible, i.e., which disintegrate into fragments small enough to not be a hazard to personnel. However, a competing consideration exists for limiting the size of the produced particles.
For some hunting and other outdoor shooting activities, there may be a desire for the projectiles to be sufficiently ductile so that the projectiles, or at least the nose portions thereof, expand or “mushroom” so as to increase the diameter of the projectile's wound path, thereby increasing trauma and lethality. In these applications, it is therefore advantageous that at least the nose of the bullet or other projectile, if not the entire projectile, be formed from a relatively ductile material. A further consideration, although not a requirement, when designing bullets and other projectiles for use in outdoor shooting activities is to select a composition that limits “sparking.” Sparking refers to the tendency of a bullet to create sparks when it strikes a hard object, as these sparks may lead to fires. An example of a material that may exhibit undesirable sparking is steel.
Thus, alternative projectile materials have been pursued. However, finding commercially viable lead-substitutes and methods for forming firearm projectiles from these compositions has not been an easy task. Previously proposed lead-substitutes include steel, copper, tin, and bismuth, which are all much less dense than lead, as well as tungsten and its alloys, which are denser than lead. Firearm projectiles formed from these materials all have been utilized as substitutes for lead projectiles in firearm ammunition, with differing degrees of success. However, projectiles made from each of these materials suffer from disadvantages. As examples, these projectiles may damage a barrel of a firearm, may not produce desired ballistic properties (such as a shot pattern, a shot velocity, a shot penetration, and/or a shot trail when fired from a shotgun), and/or may be expensive to manufacture.
Thus, there exists a need for improved projectiles that may meet environmental and/or wildlife regulations regarding toxicity while also being economical to manufacture and producing desired ballistic properties.
Zinc is a particularly promising alternative to lead firearm projectiles by virtue of its environmental and hygienic safety, castability, and relatively low cost. Zinc alloys also generally are known for having relatively high strength and corrosion resistance, if properly formed. However, prior attempts to utilize zinc alloys to form firearm projectiles arguably have not lived up to this promising potential.
Many previous attempts to utilize zinc in nontoxic bullets have employed powder-metallurgical technologies in which metal powders are compacted at high pressures into desired shapes and sizes in closed dies, optionally followed by sintering to at least partially fuse the powder particles together. Such approaches, while marginally successful, have proven to be relatively expensive.
Early attempts to produce useful articles other than ammunition from cast zinc alloys met with various levels of success, due primarily to problems associated with the presence of even slight concentrations of impurities. For example, the presence of lead, even in amounts less than 0.01 wt % (weight percent) may result in a detrimental condition referred to as “zinc pest” which causes intergranular corrosion/oxidation, surface blisters, spalling and, eventually, complete disintegration of cast articles. Significant improvements in zinc refining by the New Jersey Zinc Company (circa 1929) resulted in high-purity zinc (99.99%) which, in turn, made successful alloy development possible throughout the world. One family of such alloys, originally developed in Germany, is referred to as “Zamak” or “Zamac.” Zamak alloys all have a common concentration of approximately 4 wt % aluminum, with various intentional additions of magnesium, copper, and nickel. Table I presents chemical compositions (per ASTM B86/castings) for several Zamak alloys currently in use.
TABLE IChemical Compositions of Zamak AlloysAlloywt %AlCuMgPbCdSnFeNiZamak-min.3.52.6.025—————2max.4.32.9.05.005.004.003.1—Zamak-min.3.5—.025—————3max.4.3.25.05.005.004.003.1—Zamak-min.3.5.75.03—————5max.4.31.25.06.005.004.003.1—Zamak-min.3.5—.005————.0057max.4.3.25.02.003.002.001 .075.02 
Zamak-3 is probably the most the common Zamak alloy, and is the standard to which other Zamak alloys are compared. Zamak-3 is castable and has long-term dimensional stability.
Zamak-2 has a composition similar to Zamak-3 with the addition of about 3 wt % copper. The additional copper increases strength by about 20% relative to Zamak-3. Zamak-2 has the greatest strength of all the common Zamak alloys. Over time, it retains its strength and hardness better than the other common alloys. Nonetheless, over time, it becomes more brittle, less elastic, and shrinks.
Zamak-5 has a composition similar to Zamak-3 with the addition of about 1 wt % copper. Zamak-5 has an increased strength (by approximately 10%), hardness, and corrosive resistance relative to Zamak-3. However, Zamak-5 has reduced ductility and dimensional stability relative to Zamak-3.
Zamak-7 has a composition similar to Zamak-3 but with less magnesium, stricter control of impurities, and added nickel. Zamak-7 has increased fluidity and ductility relative to Zamak-3. The added nickel contributes to reduced intergranular corrosion.
Mechanical properties of several Zamak alloys are summarized in Table II. Ultimate tensile strength, yield strength, and shear strength are expressed in megapascal units (MPa) and may equally well be expressed in other units such as kilo-pound force per square inch (ksi; 1 MPa≈0.145 ksi). Percent elongation at fracture (% elongation) is a measure of ductility and is used generally as a proxy to characterize ductility. Hardness is characterized with the Brinell indentation hardness test and values are expressed with the Brinell hardness number (BHN).
TABLE IIMechanical Properties of Zamak AlloysZamak-2Zamak-3Zamak-5(aged)(aged)(aged)Zamak-7Ultimate Tensile397268331285Strength, MPa(331 aged)(270 aged)Yield Strength, MPa361208295285% Elongation6%6.3%3.6%14%(16% aged)(13% aged)Shear Strength, MPa317214262214Hardness, BHN130 97114 80(98 aged)
An example of an attempt to utilize Zamak alloys for firearm projectiles is U.S. Pat. No. 5,535,495 to Gutowski, the disclosure of which is hereby incorporated by reference for all purposes. The '495 patent discloses methods for producing one particular type of zinc alloy bullet; namely, a frangible pistol bullet designed and intended for use in indoor ranges, such as those used by law enforcement officers and civilian target shooters. Several problems were encountered with bullets produced according to the disclosure of the '495 patent. A primary problem is that the produced bullets, which were produced from Zamak-3, did not fragment into particles small enough to meet the current Federal Law Enforcement Training Center (FLETC) recommendation for a maximum bullet (or bullet jacket) fragment weight of 5 grains (0.324 gram). Furthermore, the '495 patent discloses a required “sizing” operation, in which the die-cast bullet precursors are mechanically deformed after production to reduce the precursors to final dimensions and tolerances. Such “sizing” operations not only add cost to the finished bullets, but also have the potential to introduce both internal and external defects (e.g., cracks, seams, laminations, etc.) into the bullet. Some of these defects, if undetected, could have serious consequences, such as causing bullet fragments to become lodged in the gun barrel as obstructions. Subsequent rounds in a repeating weapon then have the potential to cause the firearm to fail, such as by bursting the receiver and/or barrel. Another concern with the bullets of the '495 patent is that no consideration was made for the fact that the zinc die-cast alloys utilized in the '495 patent inherently exhibit “aging” phenomena. Specifically, such copper-containing zinc alloys may exhibit measurable dimensional shrinkage (e.g., 0.1% during the first month) after formation. For cast parts requiring close dimensional tolerances, certainly including bullets, this shrinkage is not acceptable.
An example of an attempt to utilize cast zinc and zinc alloy wires to form firearm projectiles is U.S. Pat. No. 5,852,255 to Hallis et al., the disclosure of which is hereby incorporated by reference for all purposes. The '255 patent discloses frangible bullets formed from cast zinc wires that are swaged together and heated, but not sintered. The '255 patent discloses that the wires must be formed from 95 wt % zinc, and preferably from 99-99.99 wt % zinc.
Other prior attempts to form firearm ammunition from conventional zinc alloys produced unsuitable projectiles, such as due to “zinc pest.” Zinc pest refers to the demonstrated incompatibility between lead and zinc, with even 0.01 wt % lead resulting in an unsatisfactory zinc alloy. This lead may be introduced, for example, when zinc alloys and/or projectiles are cast from equipment that has been used for casting lead and/or articles formed from lead.